This invention relates to the measurement of tactile sensation, and more particularly to measurement of both a minimum force to elicit touch sensation and a minimum force to elicit pain sensation in humans, and thus is useful for the measurement of skin irritation in humans.
The present invention is applicable to measuring irritation on various human tissues for example, the oral mucosa, and the like. Journal of the Society of Cosmetic Chemists, Volume 39, pages 315-320 describes the effect irritation has on the relationship of the touch and pain thresholds to one another. Other prior art, Semmes, J., Weinstein, S., Ghent, L. and Teuber H. L. (1960) Somatosensory Changes after Penetrating Brain Wounds in Man, particularly at page 61, Table B-1, describes the Semmes-Weinstein Pressure Aesthesiometer (trade name used by NCRL (NeuroCommunication Research Laboratories, Inc.), Danbury, Conn.) also generally referred to in the art as Semmes-Weinstein Monofilaments (as sold by North Coast Medical, Inc. of Campbell Calif.). This esthesiometer is a well known device to measure tactile thresholds. It comprises a set of several plastic filaments of constant length with various diameters that result in differing applied maximal forces across the different filaments. An alternative conventional device, e.g., such as the Cochet-Bonet esthesiometer, which is now out of favor, employs a single constant diameter filament which varies in length to control force.
For the purposes of measuring skin irritation according to the model of Weinstein, Drozdenko, and Weinstein (1988), Journal of the Society of Cosmetic Chemists, 39, 315-320., for example, it is necessary to measure the thresholds of touch and pain as precisely as possible. In addition, it is necessary to measure the thresholds as rapidly as possible both: (1) to prevent subject fatigue from influencing results, and (2) to obtain a measure of the potentially time-varying level of irritation. Each monofilament is applied to produce a touch or pain sensation depending on the force applied, the subject's threshold and the peculiar characteristics (i.e., whether the tip is flat, sharp or rounded) of each monofilament. The conventional esthesiometer, however, was not reliable, and hence not valid, for producing constant pressure stimulation. That is, the conventional esthesiometer caused pain beyond what would be expected for the applied pressure; this point will be expanded upon below. Moreover, this property of unexpected pain in the prior art is not consistent, but rather is probabilistic, which means that the prior art stimulus characteristics can not reliably form the basis of an evaluative model, such as the aforementioned irritation model.
Specifically, with the conventional esthesiometer device, two types of errors occur during testing on a probabilistic basis due to the characteristics of the apparatus. Of the two aforementioned types of errors, one type of error is filament slippage. The filament may skid across the surface of the skin of the subject instead of contacting and holding to one site. This error is usually obvious to the tester and can be compensated for the most part by restimulating the site as before but without slipping. However, retesting slows the evaluation process and potentially confuses the subject with a different stimulus sensation, that of a monofilament slipping (see FIG. 4). In testing, this type of error occurred for up to about 5% of the stimulations with the conventional device, but not so with the device of the present invention because of its unitary, textured tip (see FIG. 4), as will become apparent from the detailed description hereinbelow.
The second type of error is filament twisting, which is not usually obvious to the tester, and which may occur very frequently. For example, using the prior art device, the full area of the filament tip may not contact the skin being tested, with only a crescent-shaped edge making contact. If only a crescent-shaped portion, rather than the total cross-sectional area, is in effective contact with the skin, then the applied pressure may not be appropriate. What is thus now recognized as a twisting error, was formerly considered by some to be the normal mode of application, which indicates that the twisting-type error is very common. It is apparent that often only the edge of the filament is pressed into the skin. What was previously called a crescent-shaped edge is perhaps better described as the edge of a right cylindrical solid (See FIG. 1A, conventional tip). Filament twisting causes a different face (or a series of different faces, depending upon the degree of twisting) of the stimulation device (monofilament) to be presented (See FIG. 1C, a twisting error, in relation to FIG. 1B, a conventional presentation). Specifically, instead of the flat surface that may be assumed to be stimulating the target site (conventional presentation), the sharp edge of the stimulating device is presented at some unknown and variable angle (a twisting presentation). The result is that the stimulus is more likely to be perceived as painful, rather than as a pressure stimulus (see FIGS. 5 and 6). This second type of error (twisting error) makes the measurement of pain thresholds much less valid, often lowering the actual value that would have occurred. Further, this second type of error also may affect touch thresholds, by changing the nature of the stimulus perceived from merely pressure to sharp pricking, and, hence, pain.
Prior art attempts to alter the contact tip include: (1) a small plastic disk added to the contact tip (FIG. 7A)--attempted around 1955; and (2) a ball-like structure added to the contact tip (FIG. 7B). Adding material to the tip of the filament to standardize the stimulus, however, changes the applied force levels of the low-level force monofilaments. Furthermore, material is not easily added to the tip of a filament, and the added material tends to break or to fall off with repeated stimulations.
The entire contents of all of the publications identified herein and/or listed in the listing of References at the end of this specification are incorporated herein by reference.
The object of the present invention is to solve the above-described problems in the prior art, and specifically to solve the two errors which occur in the prior art devices, by the creation of a stable, functional easy to manufacture tip which better engages the skin of the subject.